Spatial light modulators, also commonly referred to as light valves are useful in many different fields. One particular field in which these devices have made an impact is the printing industry. Light valves are used in computer-to-plate imaging devices for modulating the illumination produced by a laser in order to imagewise expose a printing plate. In the imagewise exposure of printing plates pixel size and resolution are important parameters. Computer-to-plate systems make great demands upon the performance of light valves. The limits on optical power handling, switching speed and resolution are continually under pressure due to the operational demands of the printing industry. The most common lasers used for plate imaging have near-infrared wavelengths.
Light valves, or linear and two-dimensional arrays of light valves, are typically employed to produce a large number of individually modulated light beams.
Another field that stands to benefit from this technology is that of optical communications where there is a need for devices that may be used to switch, modulate, or process light signals.
One particular subset of light valves operate by controlling the reflection of an incident light beam from a micro-miniature (MEMS) deformable mirror. The term MEMS (Micro-Electro-Mechanical Systems) describes technology that forms mechanical devices such as mirrors, actuators or sensors in a substrate. MEMS devices are typically formed by selectively etching a semiconductor substrate such as a silicon wafer. Prior art MEMS light valves can be generally divided into three types:
a. cantilever or hinged mirror type light valves which re-direct a light beam when the mirror is tilted. A well-known example in this category is the Digital Micromirror Device (DMD) developed by Texas Instruments of Dallas, Tex.;
b. membrane light valves where a flat membrane is deformed into a concave or spherical mirror, thus changing the focal properties of the light beam; and,
c. grating light valves which diffract the light by forming a periodic physical grating pattern in a reflective or transparent light valve substrate. A well-known example in this category is the Grating Light Valve developed by Silicon Light Machines of Sunnyvale, Calif. and described in Bloom, Proc. SPIE—Int. Soc. Opt. Eng. (USA) 20 vol. 3013 p.165-p.171.
Considerable effort has been invested in the development of MEMS light valves. Significant technical advances have been made, particularly in improving the fabrication processes to obtain better yields. However, a number of central limitations remain in respect of MEMS devices.
A major disadvantage of the hinged or cantilevered mirror type devices is the comparatively slow response time for mirrors any larger than a few square μm in area. These devices operate by tilting a small mirror to deflect an incident beam. Typically, response times are of the order of 10 microseconds. This is due to the low natural frequency of a cantilever mirror and the large deflection required to provide sufficient spatial separation between a deflected and undeflected beam. Typical cantilever mirrors are between 5 and 10 microns long and require the tip to move between 1 and 5 microns in order to deflect the light through an angle of 10 degrees.
U.S. Pat. No. 4,441,791 to Hornbeck describes a membrane light valve. Membrane light valves have the advantage of somewhat faster response times. However, they are difficult to fabricate. The membrane is supported around its periphery making it difficult to form the cavity under the membrane by micromachining, which is the most cost effective fabrication method for light valves.
FIGS. 1a, 1b and 1c schematically depict three prior art modes of operation of deformable mirror devices of the deflection type. FIG. 1a shows a tilting mirror device having a rigid mirror 10, which remains essentially planar while it tilts about axis 12, typically on torsion hinges (not shown). FIG. 1b shows the simple cantilever type of elongate ribbon 14, which has considerably greater length than width and flexes about a transverse axis 16. Ribbon 14 is attached at one end to fixture 18. FIG. 1c shows a deformable mirror device of the type described in U.S. Pat. No. 5,311,360 to Bloom. This device has a ribbon 20 attached to fixtures 24 at ends 26 (one fixture not shown for the sake of clarity). Ribbon 20 can be flexed into a concave shape about axis 22.
All of the movable mirror elements depicted in FIGS. 1a, 1b and 1c share the problem of relatively low natural frequencies. This results in poor response times. The natural frequency of the element 30 shown in FIG. 1c may be improved by making the ribbon shorter but this makes the element more sensitive to the alignment of the incident light and requires increasingly higher voltage to actuate.